Quadrupole magnet with reduced lateral dimension



Nov. 8, 1966 G. T. DANBY ETAL QUADRUPOLE MAGNET WITH REDUCED LATERAL DIMENSION Filed Sept. 5, 1964 5 m WWW 5 m W m N 4 NW. 0 mm 4 o0 G M B R omN Q N G ,3 mm mm 6 NY 4N mm wm w m .80 OV o llllllll IHIIIIIII l O nmm United States Patent 3,284,744 QUADRUPOLE MAGNET WITH REDUCED LATERAL DIMENSION Gordon T. Danby, Wading River, and John W. Jackson, Medford, N.Y., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Sept. 3, 1964, Ser. No. 394,369 7 Claims. (Cl. 335-210) The present invention relates to a quadrupole magnet with reduced lateral dimension and low 'fringin-g fields.

The Alternating Gradient Synchrotron (AGS) located at the Brookhaven National Laboratory utilizes a series of combined tunction quadrupole focusing and dipole bending magnets, which keep the proton beam focused and bent around a circular path.

'Ilhe purpose of a quadrupole magnet is to vfocus beams of charged particles. A single quadrupole magnet focuses particles in one plane and defocuses them in a plane perpendicular to the cEocusin-g plane. However, a pair of quadrupoles with their focusing planes alternated can be set up to focus in all planes.

Either the primary proton beam, or beams of secondary particles originating in target inside the machine can be taken away from the machine for experimental observations. Especially tor the case of secondary particles it is desirable to make the beam angle with respect to the AGS as small as possible since the distribution of secondary particles produced is very sharply peaked in the forward direction. Conventional quadrupole magnets, such as these used at the AGS are too wide to be located close enough to the AGS to focus small angle beams properly.

It has been suggested that the yokes on existing quadrupoles be shrunk in size, however, study reveals that for any significant gain fringing fields will become large, and a basic condition for a magnet near to the AGS is that it have a small fringing field so as not to disturb the orbits of the protons circulating in the machine. This fringing field can be partially improved by shielding, but still only marginal gains can be made.

The present invention olves the above-mentioned problem by providing a novel quadrupole magnet with a sharply reduced lateral dimension. In a unique construction of this magnet configuration it has been made possible to reduce the width of the magnet to the point where a Sub-- stantial saving in this type OLf space is obtained. An additional feature of the invention is that it produces neg. ligible fringing fields which if present in sufficient amounts would limit the usefulness of the magnet.

It is thus a first object of this invention to provide a quadrupole magnet of reduced lateral dimension.

It is another object of the invention to provide a quadrupole magnet of reduced size and uniform magnetic field.

Still another object of the invention is a quadrupole magnet of reduced width and fringing fields.

Other objects and advantages of this invention will hereinafter become obvious from the following description of a preferred embodiment of this invention taken with the accompanying drawing in which:

FIG. 1 is an elevation view of a conventional quadrupole magnet such as those used in the AGS;

FIG. 2 is an elevation view of a simplified version of the conventional magnet shown in FIG. 1 in order to demonstrate the principles of this invention;

FIG. 2a is an isometric view of a typical pole piece; and

FIG. 3 is an elevation view of a preferred embodiment of this invention.

3,284,744 Patented Nov. 8, 1966 Referring to FIG. 1, there is illustrated a quadrupole magnet 2 which is in use at the AGS. Magnet 2 is essentially a solid piece 4 of ferrous material (although it may be assembled from separate parts for ease of manufacture) with a central opening 6 having curved pole surfaces 8 and coils 9 which are schematically illustrated.

A simplified version of the AGS quadrupole magnet is illustrated in FIG. 2 showing magnet 10 consisting of four iron pole pieces 12, 14, 16 and 18 assembled in the manner illustrated forming an air gap 20 and provided with pairs of copper windings 21, 22, 23 and 24 wrapped around pieces 12, 14, 16 and 18 respectively. Structural members 25a, 25b, 25c and 25d of iron material hold pole pieces 1218 together, while iron yokes 26, 28, 32 and 34 are assembled as shown to complete the necessary magnetic circuits as is well known in the art. Pole pieces 12, 14, 16 and 18 are identical in size and shape, and have the appearance shown for piece 16 illustrated in the isometric view shown in FIG. 2a. Typical dimensions are A:2 ft; B=6", and C:6". A typical closed circuit 'for magnetic flux lines would be through pole piece 18, yoke 26, pole piece 12 and air gap 20. One of the principal purposes Olf this invention is to reduce the width of magnet 10 shown by dimension D.

As will be seen from FIG. 3, this has been accomplished by the deceptively simple rearrangement of yokes 26- and 32 in FIG. 2. In FIG. 3, the improved quadrupole magnet 40 consists of similar pole pieces 42, 44, 46 and 48 with extra large yokes 52 and 54 located as illustrated. The phantom lines indicate what might be the separation lines between yokes of the size in FIG. 2 which could be stacked in the FIG. 3 arrangement. Across pole pieces 44 and 46 is a member 56 for structural purposes, and similarly a member 58 is located across pole pieces 42 and 48. Similar coils 61, 62, 63 and 64 complete the arrangement. The new width of magnet 40 is designated by the dimension D and it will be seen from the pole pieces and spacing as shown in FIG. 2 there will be a substantial reduction in width. The minimum thickness of the yokes, uch as dimension F for yoke 52, would be enough to decrease the fringing fields to a tolerable value for a particular application.

This would appear to give large fringing fields on the horizontal center plane outside the magnet. In addition, the fact that the four-fold symmetry has been apparently destroyed might be thought to produce a very asymmetrical magnetic field, resulting in large aberrations in the icensing properties. Both of these conclusion prove not to be the case, the point of. this invention being the recognition that, provided the tour pole and tour coils are completely symmetrical, and provided the poles are well designed so that the base of [the poles make a negligible contribution to the total magnetic reluctance of the circuit, the four-fold symmetry still holds from a magnetic point of view. That is, the location of the return yokes (52 and 54 in FIG. 3) is irrelevant. Phantom line M shows a magnetic circuit, which produces a field identical to that produced by a conventional quadr-upole. The two coils shown in the topand bottom of the aperture in FIG. 3 are driving the field around phantom line M This crosses two gaps, but twice the magnetomotive force is pushing. The two coils in the side slots can be considered to be cancelling out the field in the slots so that at the location of the vertical holding members (56 and 58), the fringing field is essentially zero. Tests prove both these contentions to be correct.

Thus it is seen that there has been provided a unique quadrupole magnet arrangement in which a substantial reduction in width is brought about without sacrificing efiiciency and effectiveness. While only a preferred embodiment of the invention is described it is understood that the invention is not to be limited lthereby but is to be defined only by the scope of the appended claims.

We claim:

1. A quadrupole electro-magnet of narrow dimension comprising:

(a) four pole pieces assembled in spaced relationship to form the corners of a rectangle and a central air p;

(b) coils wrapped around each of said pole pieces to carry electric current to establish magnetic fields and fluxes in said pole pieces;

(c) yoke members out magnetic material across two opposite pairs of pole pieces to establish a single magnetic fllllX path passing through each of said pole pieces and yokes and through the central air gap, the lateral dimension of said magnet Without yoke members: being relatively small.

2. The magnet of claim 1 in which the thickness of the yoke members is sufficiently large to limit fringing magnetic fields to an acceptable low level.

3. The magnet of claim 2 in which thin structural members are used to hold said pole pieces in said spaced relation.

4. A quadrupole electro-magnet of narrow dimension comprising:

(a) means for establishing four magnetic poles in rectangular, spaced relationship and a central air gap; and

(b) means for establishing a single closed fi uX path through said poles and passing twice through said central air gap.

5. The magnet of claim 4 in which (b) includes yoke members disposed across two opposite pairs of saidpoles along one lateral dimension of said magnet.

6. The magnet of claim 5 in which (a) includes electric conductors Wrapped around each one of said poles, each of said poles consisting of iron pole pieces.

7. The magnet of claim 6 in which all of the pole pieces are substantially identical.

No references cited.

BERNARD A. GILHEANY, Primary Examiner.

GEORGE HARRIS, Examiner. 

1. A QUADRUPOLE ELECTRO-MAGNET OF NARROW DIMENSION COMPRISING: (A) FOUR POLE PIECES ASSEMBLED IN SPACED RELATIONSHIP TO FORM THE CORNERS OF A RECTANGULAR AND A CENTRAL AIR GAP; (B) COILS WRAPPED AROUND EACH OF SAID POLE PIECES TO CARRY ELECTRIC CURRENT TO ESTABLISH MAGNETIC FIELDS AND FLUXES IN SAID POLE PIECES; (C) YOKE MEMBERS OF MAGNETIC MATERIAL ACROSS TWO OPPOSITE PAIRS OF POLE PIECES TO ESTABLISH A SINGLE MAGNETIC FLUX PATH PASSING THROUGH EACH OF SAID POLE PIECES AND YOKES AND THROUGH THE CENTRAL AIR GAP, THE LATERAL DIMENSION OF SAID MAGNET WITHOUT YOKE MEMBERS BEING RELATIVELY SMALL. 